System for converting a variable direct-current potential to an alternating current of controlled phase and amplitude



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S. WALD Filed Dec. 31, 1946 CONTROLLED PHASE AND AMPLITUDE Z /"mum IFJIC T/ON F/L fl! POTENTIAL TO AN ALTERNATING CURRENT OF SYSTEM FOR CONVERTING A VARIABLE DIRE Oct. 10, 1950 fawn/r752 earn/r run- Patented Oct. 10, 1950 srs-mu roa coma'rnzo a vamsnu: m-

OT-CURRENT POTENTlALTOANAL-' TEBNATING CURRENT OI CONTROLLED. PHASE AND AMPLITUDE Sidney Wald, Philadelphia, Pa, assignor to Radio Corporation of America, a corporation of Dela- Application December 31. ms, ScrialNo. mm

1 7 Claims.

This invention relates to systems for controlling the phase and amplitude of an alternating current by the relative variations of D. C. control potentials and has particular utility in the remote control of the direction of rotation of alternating current motors and in follow-up systems where a condition of unbalance controls the movement of a follow-up device, the movement of which tends I to restore the initial condition of balance.

The use of a synchro-generator and synchromotor for controlling the rotation of a remote object is well known. Frequently it is desirable to control the rotation of an alternating current motor by means of a D. C. control potential which may be, for example, produced by the movement of a controlling mechanism or which may result from a change in the electrical balance of two D. C. voltages. It is therefore necessary to provide means for converting the variation of D. C. potential into an alternating current which reverses in phase in accordance with the sense of the initial change and which corresponds in amplitude with the degree of change.

It is therefore a primary object of this invention to provide an improved method of and means for converting a change in D. C. potential into an alternating current which changes in phase and amplitude in accordance with the direction and amplitude of the change. a

It is a further object of this invention to provide an alternating current of reversible phase for the control-of a motor in accordance with changes in the relative amplitudes of two D.'C. control potentials.

In a copending application of J. W. Conklin, Serial No. 476,903, filed February 24, 1943, now Patent Number 2,423,228 issued July 1, 1947, and assigned to the same assignee as'the present invention, there is described a remote control system of the general type claimed in the present application. In accordance with the aforemen-,

tioned copending application, D. C. control potentials which are variable in mutually opposite directions from a normally negative value are applied to' the grid electrodes of two pairs of thermionic tubes whose plate electrodes are energized by A. C. voltages. By means of output transformers the plate impedances of the 'two pairs of tubesare reflected into a motor circuit to control the direction of rotation of the motor. While such a system has many practical advantages, the use of negative D. C. control voltages introduces certain limitations in its. utility which it is the purpose of this invention to overcome. Also, in many cases the size and weight of the 2 output transformers used in the motor circuit are prohibitive, as, for example, where such a system is to be used in aircraft where spac and weight are at a premium.

It is therefore a further object of this invention to provide a, remote control or follow-up systemin which no power transformers are required, and, further, which will operate satisfactorily when positive control potentials are applied to the tube. This is important for' the reason that where the system is employed to control a system in which the actuating change is already in the form of a positive variable D. C. voltage, it is not necessary to convert th control voltage to one of negative potential. s

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention both as to its organization and method of operaing description when-read in connection with the accompanying drawings, in which a Figure l is a circuit diagram of a remote control system in accordance with this invention, and

Figure 2 is a circuit diagram of a servo system utilizing this invention.

Two independent 1). C. control voltages may be obtained by various means. The general case where they are independently adjustable is illustrated in Fig. l, the two voltages being produced by means of a battery 3 and two potentiometers 5 and I. The negative terminal of the battery is connected to ground. The contact arms of the two potentiometers are connected to the grid electrodes of a pair of thermionic discharge devices 9 and II respectively, through series resistors l3 and II. An alternating voltage of suitable amplitude and preferably of cycle frequency is impressed on the-grid electrodes by means of two capacitors I1 and I! which are connected to the two terminals of the secondary of a small voltage transformer 2 I A potentiometer 23 is connected across the transformer secondary,

.is derived from the transformer it may be very small. The cathode electrodes of the two tubes are connected together and through a resistor 25 to ground. This common cathode resistor conaccuse being connected to the 60 cycle line voltage. The

plate electrodes of the two thermionic discharge devices 9 and H are connected directly to a suitable source of D. C. potential which may be supplied by a battery, D. C. generator or the like.

The contact arm of potentiometer It is iirst adjusted to balance the amplitude of the 2 A. C. voltages which are impressed on the grid electrodes of the two tubes. While a center tapped transformer secondary may be employed, the arrangement shown provides a convenient method for insuring that the initial adjustment of the circuit is correct. since for balanced operation the two A. C. voltages should be equal in amplitude and opposite in phase.

As is well known the reversible motor 33 requires the application of alternating currents which are in phase quadrature to the two windings SI and 35. This may be accomplished by inserting a suitable phase shifting device in circuit with winding 38, or, as in the present case, it is possible to take advantage of the inherent phase shift produced in the circuit to provide the necessary quadrature relationship. The second harmonic rejection filter 21 inherently produces a certain phase shift at 60 cycles, assuming that the normal line frequency is 60 cycles, the actual degree of phase shift depending upon the circuit constants chosen. It has been found that a rejection filter suitable for the purpose produced a phase shift of approximately '10. In order to provide the additional phase shift necessary, use is made of the capacitors I1 and II which are preferably included in the grid circuit of the two' tubes to prevent the transformer from short circuiting the D. C. potentials applied by the control device. By suitably selecting the values of resistor I3 and capacitor l1 and likewise the values of resistor I5 and capacitor II. the additional phase shift may readily be obtained.

One of the features of this invention lies in the fact that the system responds to the difference between the amplitudes of the two control potentials. That is, the position of the contact arm of either potentiometer I or I may be taken as.

a reference voltage, and the device will then operate to rotate the motor in one direction or the other, in accordance with the relative amplitude able degree and applied to the motor to cause its rotation in a given direction. If, however. the D. C. voltage applied to the grid of tube I is less positive than that applied to the grid oi tube II, the output voltage will be of the opposite phase and the motor will therefore rotate in the opposite direction.

Systems previously known have required the I application of negative D. C. control voltages to the control tubes, as is the case in the copending application referred to above. In the earlier systems one tube or the other was biased oi! to provide the discriminatory action. It is frequently desirable to control the remote operation of the motor in response to a D. C. voltage which is positive. The present invention permits direct use of such a positive voltage by reason of the fact that the use of the common cathode output resistor 2| makes the system 100% degenerative. That is, all the output voltage is applied to the cathode electrodes. and this affects the grid-to-cathode voltage of the tubes in a sense which directly opposes the applied voltage. As a result, the grid can never be driven positive with respect to the cathode, and it is therefore possible to operate the tubes with large positive D. C. control voltages. This provides a much larger degree of stability than has heretofore been obtainable, both with respect to variations in the characteristics of the tubes themselves and to variations in the line voltage. or the D. C. potential applied to the anode electrodes. A further advantage of the systemv illustrated lies in the fact that the tubes are operated over the linear portion of their input-output characteristics, so that the response is more linear with respectto the change of amplitude of the D. C.

control potential than has heretofore been poscuit also eliminates the need for push-pull output transformers which have heretofore been required, thus effecting a considerable saving in the size and weight of the equipment;

A still further advantage of the arrangement shown is that the voltage applied to either grid electrode may be considered as the reference voltage, and the operation of the device is then effected by comparing the other'voltage to it. That is, the condition of balance may be determined by adjusting the value of either one of the two is, when the two D. C. control potentials'are,

equal in amplitude, each tube passes alternating current of a given amplitude,. but of opposite phase, through the common cathode resistor 2!. As a result, only a direct voltage is developed across the resistor and the motor 33 is not eners z If the potential applied to the grid of tube 0 becomes more positive than that applied to the grid of tube II, the component of current in the output resistor 25 of the phase controlled by tube 0 will be greater than the component of current in the opposite phase, and an alternating output voltage will result. This voltage is passed through the second harmonic rejection filter, which eliminates the second harmonic since such current would serve no useful purpose in energizing the motor. The A. C. voltage is amplified to a suit- D. C. control potentials independently of the existing value of the other. A practical application for such an arrangement is illustrated in Pig. 2. to which reference is now made.

, In the embodiment of the invention-illustrated in Fig. 2, it is assumed that thermionic discharge device 31 is the output or power amplifier of a radio transmitter, not shown. The plate electrode is connected to an inductance II which constitutes the primary of an output transformer. The tube is energized by a source of D. C. potential II which is assumed to be a D. C. generator subject to undesired voltage variations. The coupling between the primary is and secondary 43 of the output transformer to which the load is connected is adjustable. the degree of coupling being controlled by a suitable mechanical movement coupled to a reversible motor 41 of the type utilized in the foregoing illustration. A resistor l! is connected in the cathode circuit of the output tube 31, suitably by-paseed for radio frequenciesbya capacitor II. and connected through aseriesresistor lltothegridofathermionic discharge device I. A voltage is derived from the D. C. generator ll which i proportional to the generator outputand whose amplitude is equal to the D. C. voltage produced across resistor ll. This is accomplished, for example, by means of a voltage divider It. The low potential tap of voltage divider It is connected through a series resistor is to the grid of a thermionic discharge device ii. As in the preceding case, alternating voltage of equal amplitude and opposite phase are impressed on the two grid electrodes by means of a transformer Ii through series connected capacitors l1 and II respectively. As before, the plate electrodes of the two discharge devices and Ii are connected to'a suitable source of D. C. potential such as generator ll Output is developed across the common cathode resistor 25 and is applied'through the second harmonic rejection filter 21 and amplifier It to winding ii of motor 41. The remaining winding is is connected to the line voltage.

It is well known that, the degree of coupling for most eillclent operation of a transmitter depends upon the amplitude of the applied plate voltage. 1 desirable to compensate for this variation by a corresponding change in the coupling of the load. It is assumed, for example, that the transmitter is an aircraft transmitter in which the D. C. voltage for the operation of the transmitter is subject to inherent and undesired variations of potential. The criterion for proper couplings is that the plate current be maintained at a value which bears a. direct relation to the applied plate voltage. There are therefore two factors to be considered in determining the proper degree of Thus, if the plate voltage varies it is.

- generators which are used for dielectric or incoupling, the plate voltage and the plate current,

In accordance with the present invention these two factors are used to produce the two positive D. C. control voltages which are independently variable, the system being designed to maintain the two at apredetermined relationship. Thus, the D. C. potential across resistor 49 and the D. C. potential acrossvoltage divider 53 are initially adjusted to have equal values when the couplingis corrected for the normal operating potential. In such a condition the motor 41 remains at rest sinceno cycle voltage is developed across the cathode resistor 25. However, if the voltage developed by generator 41 decreases, the motor 41 is caused to rotate in such a direction that the coupling between the load and the transmitter output tube is reduced. As is well known, the reduction of coupling changes the plate current in the ampli- I tying tube, and this change is carried on until the plate current assumes the proper value, as determined by the voltage drop across resistor 49. When this voltage again equals the control voltage from the D. C. generator the system is restored to balance. If, however, the generator voltage increases, then the motor rotates in the opposite direction and the coupling will be induction heating. In such case the antenna load would be replaced by the load of the device being heated, which, as is well known, reflects considerable variation on the transmitter as it passes through its heating cycle.

A still further advantage of this invention lies in the utilization of a second harmonic rejection illter, particularly in follow-up systems of the type illustrated in Fig. 2. It is well known that the inclusion of a resonant circuit in a follow-up system increases instability of the type known as "hunting." Since the undesired harmonic frequencies are removed by a sharply tuned rejection filter, no resonant circuits are utilized tuned to the operating frequencies of 60 cycles and therefore the tendency to hunt is greatly reduced.

What I claim is:

1. Asystem for converting the diiference between two variable voltages to an alternating voltage whose phase and amplitude correspond to the sense and amplitude of said difference, comprising a pair of thermionic tubes having cathode, grid and plate electrodes, means for applying said variable voltages to said grid electrodes respectively in a polarity positive with respect to the cathodes; means for impressing an alternating voltage between said cathode and grid electrodes of one of said tubes and an alternating voltage of opposite phase between said cathode and grid electrodes of the other of said tubes; means for applying a D. C. potential to said plate electrodes, impedance means in circuit with said cathode electrodes, and output means connected to said impedance. 1

2. In a device for controlling the direction of rotation of a motor in accordance with the difference between two variable D.-C. voltages, the

- combination which includes a pair of thermionic tubes having cathode, grid and plate electrodes; control means for varying the D. C. potential of said grid electrodes respectively comprising means for applying each of said D.-C. voltages respectively to said grids in a polarity positive with respect to said cathodes; means for impressing an alternating voltage between said cathode and grid electrodes of one of said tubes and an alternating voltage of opposite phase between said cathode and grid electrodes of the other of said tubes; means for applying a D. C. potential to said plate electrodes, 9. common resistor in circuit with said cathode electrodes, a reversible motor of the type in which the direction of rotation is controlled by the phase of the alternating current applied to at least one winding thereof; and means for applying the alternating voltage across said resistor to the winding of said motor.

3. In a device of the character described, control means for producing two D. C. voltages which may be varied in amplitude independently; a pair of thermionic discharge devices having cathode, grid and anode electrodes; means for applying said D. C. voltages to said grid electrodes respectively in a polarity positive with respect to the cathodes; means for impressing alternating voltages of opposite phase on said grid electrodes; means for applying a fixed D. C. potential to said anode electrodes, means for whose phase is a function of the relative amplitudes of said D. C. voltages; and means for maintaining each grid electrode slightly negative with respect to its associated cathode for all values of D. C. voltage applied thereto.

4. A device of the character described in claim 3 in which said means for deriving an output voltage includes an impedance in circuit with said cathode electrodes, said impedancealso providing degenerative feedback for maintaining said grid electrodes slightly negative.

5. In a device for maintaining constant a. preestablished relationship between two conditions, each subject to variation, means for producing D. C. voltages whose amplitudes vary with changes in said conditions and which are equal' when said pre-established relationship is established: a pair of thermionic discharge devices 9 having cathode, grid and anode electrodes, means for applying said D. C. voltag to said grid electrodes, means for also impressing alternating voltages of opposite phase on said grid electrodes, means for applying a fixed D. 0. potential to said anode electrodes, means for deriving an alternating output voltage from said device whose phase is a function of the sense of variation from said relationship, and means responsive to said alternating voltage for restoring said pro-established relationship between said con- 80 a anoutwttubeandaloadvariably prising 'eoupledtheretmmeansforderivingaiirstnc.

control voltage ml to the. plate current oisaidoutput tube,asourceofD.C.vol ageior crsizingsaidtubeandbeingsubiect potential fluctuations, means for deriving a second D. C. control voltage proportional to the vices having cathode, grid and anode electrodes, means for applying said control voltages to said grid electrodes respectively, means for applying alternating voltages of opposite phase to lid grid electrodes respectively, a resistor in circuit with the cathode electrodes of said devices ior I producing an output voltage, a reversible motor energised by said output voltage, and means for y nsthecouplingoisaidloadtosaidoutput tube in a direction which compensates for any change in either of said control voltages.

SIDNEY WALD.

assurances crr The following references are of record in the die of this patent:

UNITED STATE PATINTB Article: The Amplification of Direct-Current Signals Through Alternating-Current Circuits, by '1. J; Goldsmith, DuMont Oscillographer 8,

40 Numbers 6-7, 1 (was). 

